1. Field of the Invention
The present invention relates to a surface acoustic wave device using a quartz substrate, and more particularly relates to a greatly improved a surface acoustic wave device using a surface acoustic wave substrate that is formed by laminating a piezoelectric thin film on a quartz substrate.
2. Description of the Related Art
In the past, surface acoustic wave devices have been widely used, for example, for bandpass filters of mobile communication devices. A surface acoustic wave (hereafter xe2x80x9cSAWxe2x80x9d) device has a structure in which at least one interdigital transducer (hereafter xe2x80x9cIDTxe2x80x9d) composed of at least one pair of comb electrodes is formed so as to contact the piezoelectric body.
Furthermore, various types of SAW devices using a piezoelectric thin film have also been proposed in recent years. Specifically, SAW devices using a surface acoustic wave substrate composed of a piezoelectric thin film formed on an elastic substrate such as a glass substrate and a piezoelectric substrate have been proposed.
The four types of structures shown in FIGS. 22(a), 22(b) and 23(a), and 23(b) are known as structures using a surface acoustic wave substrate formed by laminating the above-mentioned piezoelectric thin film and elastic substrate. Specifically, in the SAW device 101 shown in FIG. 22(a), a piezoelectric thin film 103 is formed on an elastic substrate 102, and IDTs 104 are formed on the piezoelectric thin film 103, while in the SAW device 105 shown in FIG. 22(b), the IDTs 104 are formed on the lower surface of the piezoelectric thin film 103, i.e., in the interface between the elastic substrate 102 and the piezoelectric thin film 103.
Furthermore, in the SAW device 106 shown in FIG. 23(a), a short-circuiting electrode 107 is formed on the elastic substrate 102, and the piezoelectric thin film 103 is laminated on top of this short-circuiting electrode 107. The IDTs 104 are formed on the piezoelectric thin film 103. In other words, the structure of the SAW device 106 corresponds to the structure of the SAW device 101 shown in FIG. 22(a) with the short-circuiting electrode 107 inserted in the interface between the elastic substrate 102 and the piezoelectric thin film 103.
In the SAW device 108 shown in FIG. 23(b), the short-circuiting electrode 107 is formed on the piezoelectric thin film 103. Furthermore, the IDTs 104 are formed in the interface between the elastic substrate 102 and the piezoelectric thin film 103. Therefore, the structure of the SAW device 108 corresponds to the structure of the SAW device 105 shown in FIG. 22(b) with the short-circuiting electrode 107 formed on the upper surface of the piezoelectric thin film 103.
FIG. 24 shows the electromechanical coupling coefficients of the above-mentioned SAW devices 101, 105, 106, and 108 in a case where the structures of these devices are only differentiated by the formation position of the IDTs 104 and the presence or absence of the short-circuiting electrode 107, and other structures are kept the same, with a ZnO thin film used as the piezoelectric thin film, and a glass substrate used as the elastic substrate.
FIG. 24 illustrates changes in electromechanical coupling coefficients with respect to the normalized thickness H/xcex of the ZnO thin film in the above-mentioned four types of SAW devices. In the present specification, H indicates the thickness of the piezoelectric thin film, and xcex indicates the wavelength of the surface acoustic wave to be excited (units are the same in both cases).
Furthermore, the solid line A indicates the results for the SAW device 101, the broken line B indicates the results for the SAW device 105, the one-dot chain line C indicates the results for the SAW device 106, and the two-dot chain line D indicates the results for the SAW device 108.
As is clearly seen from FIG. 24, larger electromechanical coupling coefficients can be obtained with the SAW devices 105 and 108 than with the SAW devices 101 and 106 by selecting H/xcex.
Accordingly, it has conventionally been thought that larger electromechanical coupling coefficients can be obtained when the IDTs 104 are formed in the interface between the glass substrate 102 and the ZnO thin film 103 in a structure in which the ZnO thin film 103 is formed on the glass substrate 102. Furthermore, the waves indicated as Sezawa waves in FIG. 24 are a higher-order mode of surface acoustic waves of the Rayleigh type.
In addition, various characteristics of the surface acoustic wave in the case of using a surface acoustic wave substrate in which a ZnO thin film is formed on a quartz substrate are described by the present inventors in IEEE ULTRASONICS SYMPOSIUM (1997), pp. 261-266 and in the research data from the 59th Acoustic Wave Device Technology No. 150 Committee Meeting (1998) of Japan Society for the Promotion of Science, pp. 23-28 (hereinafter referred to as xe2x80x9cReference 1xe2x80x9d). These characteristics are described with reference to FIGS. 25(a), 25(b), and 26. In this prior art, it is theoretically and experimentally confirmed that a surface acoustic wave substrate with the temperature coefficient of frequency (TCF) of zero can be obtained by forming a ZnO thin film that has a negative value of the TCF on a quartz substrate with a cut angle and propagation direction which are such that the TCF has a positive value.
Furthermore, the theory in this Reference 1 is based on IEEE Trans. Sonics and Ultrasonic. Vol. SU-15, No. 4 (1968), page 209.
FIG. 25(a) shows the ZnO film thickness dependence of the TCF of the SAW device shown in FIG. 22(a) using the quartz substrate described in Reference 1 mentioned above, which is made of a 29xc2x045xe2x80x2 rotated Y-cut 35xc2x0 X propagating plate, and which has the Euler angles of (0xc2x0, 119xc2x045xe2x80x2, 35xc2x0). FIG. 25(b) shows the ZnO film thickness dependence of the TCF of the SAW device shown in FIG. 22(a) using the quartz substrate described in Reference 1 mentioned above, which is made of a 42xc2x045xe2x80x2 rotated Y-cut 35xc2x0 X propagating plate, and which has the Euler angles of (0xc2x0, 132xc2x045xe2x80x2, 35xc2x0). Furthermore, FIG. 26 shows the electromechanical coupling coefficients of the Rayleigh waves and the Sezawa waves constituting the spurious waves of the SAW devices that use a ZnO thin film as the piezoelectric thin film and a quartz substrate as the elastic substrate. The solid lines A through C in FIG. 26 indicate the electromechanical coupling coefficients of the Rayleigh waves in the SAW device structures shown in FIGS. 22(a), 22(b), and 23(a), respectively, while the broken lines Axe2x80x3, Cxe2x80x3, Dxe2x80x3 indicate the changes in the electromechanical coupling coefficients of the Sezawa waves constituting the spurious waves in the SAW devices having the structures shown in FIGS. 22(a), 23(a), and 23(b), respectively.
It is seen from FIGS. 25(a) and 25(b) that the TCF becomes zero by selecting the normalized thickness of the ZnO film in the SAW device of FIG. 22(a).
Table 1 below shows the comparison between the SAW device of FIG. 22(a) (Al/ZnO/quartz laminated structure), which is described in the above-mentioned prior art, and a conventionally known SAW device having a favorable TCF.
It is seen from FIG. 26 and Table 1 that with the SAW device of FIG. 22(a), approximately 1% of the electromechanical coupling coefficient K2 is obtained, which is larger than in the case of the ST-X quartz substrate or La3Ga5SiO14 substrate, and the acoustic velocity is lower by about 20% than in the case of the Li2B4O7 substrate, which has a comparable electromechanical coupling coefficient K2. This means that when a transversal-type SAW filter is constructed using the SAW device of FIG. 22(a), loss is lower than in the case of the ST-X quartz substrate or La3Ga5SiO14 substrate, and this SAW filter is more compact and has a lower frequency deviation caused by the temperature than in the case of the Li2B4O7 substrate.
Incidentally, it is indicated in FIG. 26 that when a ZnO thin film is used as the piezoelectric thin film, and a quartz substrate is used as the elastic substrate, the electromechanical coupling coefficient of the Rayleigh wave in the SAW device of the FIG. 22(b) is smaller than the electromechanical coupling coefficient of the Rayleigh waves in the SAW devices of FIGS. 22(a) and 23(a). This tendency is opposite of the tendency seen in cases where a glass substrate is used as the elastic substrate.
Thus, because the SAW devices of FIGS. 22(a) and 23(a) possess both a favorable TCF and a large electromechanical coupling coefficient, use of a device such as these makes it possible to enhance the performance of a bandpass filter and other surface acoustic wave devices used in a mobile communication device.
However, the problem is that even with the SAW devices of FIGS. 22(a) and 23(a), the electromechanical coupling coefficient is still not enough to sufficiently satisfy characteristics required for surface acoustic wave devices. Mobile communication systems have been shifting from the conventional analog system to the digital system, and to the code diffusion system. In an intermediate-frequency filter used in a digital system or a code diffusion system, for example, a low group delay deviation and low insertion loss are demanded. Transversal-type filters are known as bandpass filters using surface acoustic wave devices having a small group delay deviation. When a transversal-type filter is constructed using a conventional surface acoustic wave substrate, however, the electromechanical coupling coefficient is insufficient, so that the above-mentioned demands cannot be met.
In order to overcome the problems described above, preferred embodiments of the present invention provide a SAW device which uses a surface acoustic wave substrate formed by laminating a quartz substrate and a piezoelectric thin film, and which has low spurious response, superior temperature characteristics, and a very large electromechanical coupling coefficient.
According to a preferred embodiment of the present application, a surface acoustic wave device includes a quartz substrate in which the Euler angles (xcfx86, xcex8, "psgr") are such that xe2x88x9219xc2x0 less than xcfx86 less than +15xc2x0, 107xc2x0 less than xcex8 less than 125xc2x0, and xe2x88x9210xc2x0 less than "psgr" less than 15xc2x0, a piezoelectric thin film disposed on the quartz substrate, and comb electrodes arranged so as to contact the piezoelectric thin film, wherein the normalized thickness H/xcex of the piezoelectric thin film is at least about 0.05 where the thickness of the piezoelectric thin film is H, and the wavelength of the surface acoustic wave is xcex.
Preferably, the quartz substrate has the Euler angles (xcfx86, xcex8, "psgr") that are such that xcfx86 is in the range of about xe2x88x922.5xc2x0xc2x15xc2x0, xcex8 is in the range of about 116xc2x0xc2x15xc2x0, and "psgr" is in the range of about +2.5xc2x0xc2x15xc2x0.
In a specific preferred embodiment of the present invention, the normalized thickness H/xcex of the piezoelectric thin film is at least about 0.20.
In another specific preferred embodiment of the present invention, the surface acoustic wave device is configured so that the piezoelectric thin film contacts at least one of the comb electrodes on the negative side of the piezoelectric thin film.
In yet another specific preferred embodiment of the present invention, a short-circuiting electrode is provided on the piezoelectric thin film.
In another specific preferred embodiment of the present invention, the Euler angles of the quartz substrate are such that the power flow angle (PFA) of the Rayleigh wave in FIG. 6 is in the range of about xc2x12.5xc2x0.
In another specific preferred embodiment of the present invention, the Euler angles of the quartz substrate are such that the temperature coefficient of frequency (TCF) of the surface acoustic wave device in FIG. 7 is in the range of about xc2x125 ppm/xc2x0 C.
In another specific preferred embodiment of the present invention, the Euler angles of the quartz substrate are such that the temperature coefficient of frequency (TCF) of the surface acoustic wave device in FIG. 7 is in the range of about xc2x15 ppm/xc2x0 C.
In another specific preferred embodiment of the present invention, the Euler angles of the quartz substrate are such that the electromechanical coupling coefficient K2 of the Rayleigh wave in FIG. 8 is about 0.8% or larger.
In another specific preferred embodiment of the present invention, the temperature coefficient of frequency (TCF) of the piezoelectric thin film has a negative value.
In another specific preferred embodiment of the present invention, the Euler angles of the quartz substrate are such that the difference of the power flow angles xcex94PFA between the surface acoustic wave to be utilized and unnecessary surface acoustic wave in FIG. 18 is in the range of about xc2x11xc2x0.
In still another specific preferred embodiment of the present invention, xcfx86 of the Euler angles (xcfx86, xcex8, "psgr") of the quartz substrate is approximately xe2x88x9235 to +35xc2x0.
In preferred embodiments of the present invention, furthermore, a quartz substrate having the Euler angles that are crystallographically equivalent to the above-mentioned specific Euler angles of the quartz substrate may also be used.
In preferred embodiments of the present invention, the above-mentioned piezoelectric thin film is preferably formed from one type of material selected from the group consisting of ZnO, AlN, Ta2O5, and CdS.
The above and other features, elements, characteristics and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.